This invention relates to coal gasifiers, and more specifically, to a circular slag tap for a pressurized coal gasifier.
Current governmental air pollution standards limiting the level of emissions from the stacks of fossil fuel-fired power generation equipment have created an urgent need for clean burning fuels. An obvious solution to this problem is to burn fuels which are low in particulate matter and sulfur content, and to do so in a manner that will minimize NO.sub.x emissions. However, such fuels are in relatively short supply and also are relatively more expensive.
One solution to the problem, particularly in the United States where the basic proven energy reserves are predominantly coal, is the gasification of coal to produce a fuel gas suitable for firing in a steam generator and similar devices, or for firing in a gas turbine, which is part of an integrated gasification combined cycle system (IGCC). Generally speaking, coal gasification involves the reaction of coal, at high temperatures, with a gas containing oxygen, and steam to produce a gas, containing principally carbon monoxide (CO) and hydrogen (H.sub.2), which is suitable for use as a fuel gas.
The processes, which to date have been employed for purposes of achieving coal gasification, can be conveniently divided into three categories:
1. Fixed bed gasification where lump coal is supported on a grate or by other means and the flow of gas and coal may be concurrent or countercurrent; PA1 2. Fluidized bed gasification where crushed or fine coal is fluidized by the gasifying medium, giving an expanded fuel bed that can be visualized as boiling liquid; and PA1 3. Suspension or entrainment gasification where fine coal is suspended in the gasifying medium such that the fine coal particles move with the gasifying medium either linearly or in a vortex pattern.
At one time fixed bed gasifiers were felt by many to be the most attractive from the standpoint of economics. In this regard, the economically desirable features of fixed bed gasifiers were deemed to encompass the following features: their inherent relatively high carbon conversion ratio, the fact that a low producer volume is required per unit of gas production, and the fact that a minimum amount of fuel preparation is required in connection with the use thereof.
By way of exemplification and not limitation, one example of a fixed bed gasifier is that which forms the subject matter of U.S. Pat. No. 3,920,417 entitled "Method of Gasifying Carbonaceous Material", which issued on Nov. 18, 1975 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 3,920,417, there is provided a method of producing a clean low BTU fuel gas by the reaction of a carbonaceous fuel with free oxygen and steam in a downdraft fixed bed gasifier. The material to be gasified is introduced at the upper end of the fixed bed gasifier where it is preheated, dried and devolatilized by a stream of hot gas from the lower zones of the fixed bed gasifier, which has been drawn upward and which is withdrawn from the fixed bed gasifier at the upper end thereof. This upward flow of gas also serves to maintain the ignition level of the carbonaceous fuel material at a predetermined level within the oxidation zone of the fixed bed gasifier. The portion of gas withdrawn from the upper end of the fixed bed gasifier, which contains the volatiles released by the carbonaceous material, is reintroduced into the fixed bed gasifier in the oxidation zone where the temperature is sufficiently high to effect thermal cracking of the volatiles to valuable hydrocarbon compounds. Continuing, as the carbonaceous fuel material moves downward through the shaft of the fixed bed gasifier it passes successively through oxidation and reduction zones where air or some other free oxygen containing gas, and steam, respectively, are introduced into the fixed bed gasifier. The resulting reactions convert the carbonaceous fuel material and other reactants to an incandescent char matrix extending to the shaft of the fixed bed gasifier, gaseous products (the makeup thereof depending upon the level of the fixed bed gasifier), and a molten slag.
A subsequent modification in the design of fixed bed gasifiers wherein a second stage has been added in cooperative association with the fixed bed gasifier forms the subject matter of U.S. Pat. No. 4,069,024 entitled "Two-Stage Gasification System", which issued on Jan. 17, 1978 and which is assigned to the same assignee as the present application. In accordance with the teachings of U.S. Pat. No. 4,069,024, there is provided a method and apparatus for directing a low BTU content synthesis gas, as produced in a fixed bed gasifier, for example, of the type previously disclosed in U.S. Pat. No. 3,920,417, through a second stage having a serially interconnected pyrolyzer of so-called "spouting bed" design wherein the hot synthesis gas from the fixed bed gasifier is contacted by a hot carbonaceous charge and lime to utilize its high sensible heat to produce a cool, higher BTU product. Reactions take place between some of the carbon of the charge and hydrogen of the gas to increase the hydrocarbon content of the gas before it is conducted to its place of use, while the lime reacts with the sulphur to reduce if not eliminate the sulphur content of the gas. The remaining charge of char is devolatilized and passed on to the fixed bed gasifier where a hot oxidizer and steam are added thereto to produce more synthesis gas that is supplied back to the second stage, i.e., to the pyrolyzer, for reaction with the carbonaceous charge and lime.
Turning next to a consideration of entrainment, i.e., suspension, gasification, there are examples thereof to be found in the prior art. By way of exemplification and not limitation in this regard, one such example of an entrainment gasification process is that which forms the subject matter of U.S. Pat. No. 4,158,552 entitled "Entrained Flow Coal Gasifier", which issued on Jun. 19, 1979 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,158,552, a high temperature level of product gas stream is formed by burning primarily char with the existing air supply. Immediately thereafter additional char is introduced into the high temperature stream for gasification of these carbon particles. Then, following the endothermic gasification reaction which cools the gases, new fresh coal is introduced with this coal being devolatilized at relatively low temperature, thus utilizing low temperature heat. Entrained char particles are thereafter removed from the gas stream and reintroduced into the gasifier. The low temperature devolatilization of the fresh coal is achieved by gas temperatures at a level which is insufficient to effectively continue the carbon gasification process. Accordingly, more of the available heat is used for the basic purpose of the coal gasification operation, which is of course to produce gas having the maximum reasonable heating value.
Another example of an entrainment gasification process is that which forms the subject matter of U.S. Pat. No. 4,343,627 entitled "Method of Operating a Two-Stage Coal Gasifier", which issued on Aug. 10, 1982 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,343,627, an object of the invention disclosed therein is to increase the effectiveness of an entrained flow gasifier by enhancing its ability to produce a product gas having an increased heating value. Moreover, the manner in which this is accomplished is that steam is used as the conveying medium for carrying the additional coal to be injected into the reduction zone from the coal source to the reduction zone. Upon entering the reduction zone, the steam reacts with the additional coal to form carbon monoxide and hydrogen. Therefore, rather than lowering the heating value of the product gas as is the case when either air or inert gas are used as the conveying medium, the steam reacts to form additional carbon monoxide and hydrogen thereby increasing the heating value of the product gas formed in the reduction zone. Further, by using steam as the conveying medium, the possibility of an explosion in the transport line between the coal source and the reduction zone is eliminated.
Yet another example of an entrained gasification process is that which forms the subject matter of U.S. Pat. No. 4,610,697 entitled "Coal Gasification System with Product Gas Recycle to Pressure Containment Chamber", which issued on Sep. 9, 1986 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,610,697, there is provided a pressurized coal gasifier for producing a clean, particulate free fuel gas suitable for use in a gas turbine-generator or a feedstock for a methanation or other chemical process. The subject gasification system includes a pressure containment chamber surrounding an inner, water-cooled chamber wherein a gasification reaction takes place, a second pressure containment chamber surrounding a process gas heat exchanger for cooling the gas produced by the gasification reaction, and a conduit between the first and second vessels having an outer pressure containing wall and an inner water-cooled transfer duct for conducting the produced gas from the gasifier vessel to the heat exchanger. Continuing, the gasifier vessel includes a slag tap disposed at the bottom for removal of any molten slag produced by the gasification reaction. Differential thermal expansion between the pressure containment chamber and the water-cooled gasifier chamber is accommodated without loss of inter-chamber sealing by a water seal between the chambers without the occurrence of gas leakage into the annular volume.
Still another example of an entrained gasification process is that which forms the subject matter of U.S. Pat. No. 4,680,035 entitled "Two Stage Slagging Gasifier", which issued on Jul. 14, 1987 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,680,035, a two stage coal gasifier is provided that has a vertically elongated gasifier chamber for upward flow of the gas to an opening at the top. Tubes comprising the walls of the chamber form a centrally located slag tap opening at the bottom of the chamber. In the lower portion of the gasifier is a combustion section where fuel injection nozzles inject coal tangentially along with combustion supporting air to supply the heat source. At an upper elevation is a reductor section wherein additional fuel is introduced for the purpose of gasifying this additional fuel. Between the combustor section and the reductor section there is a centrally located baffle. This baffle is larger in plan area than the tap opening and is arranged so that any slag falling around the baffle will land some distance remote from the slag tap opening. The centrally located baffle minimizes the radiation loss from the combustor to the reductor, blocks the central portion of the gasifier chamber so that gas is not drawn back down from the reductor section, and forces the slag falling from the reductor to enter the combustor at a location outboard of the slag tap opening so that it has time to be heated substantially before passing to the slag tap opening.
Although gasifier systems constructed in accordance with the teachings of the various U.S. patents to which reference has been had hereinbefore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such gasifier systems to be further improved if increased usage thereof is to be realized. More specifically, there has been evidenced in the prior art a need for a new and improved slag tap system for a gasifier. To this end, there is widely held the belief that a cost effective, efficient means of generating electricity in the 1990's resides in the utilization of a coal gasifier in a combined cycle plant. Furthermore, an integral part of such a gasification process is that the gasifier converts all ash into the form of a fused ash, which is commonly referred to as slag. This slag is an environmentally acceptable waste product, which is drained from the bottom of the gasifier vessel. The temperature within the lower portion of the gasifier vessel is normally on the order of 3000.degree. F. Thus, in order to properly remove the slag from the gasifier vessel, i.e., to tap the slag, a properly designed tapping hole is necessary. In addition, special precautions are required to be taken in order to provide proper slag tapping over a wide range of operating conditions and coal ash compositions.
Moreover, there has been evidenced in the prior art a need for such a new and improved slag tap system for a gasifier that would be particularly characterized in a number of respects. In this regard, one such characteristic which such a new and improved slag tap system would desirably possess is that it be of self-supporting construction. Another characteristic which such a new and improved slag tap system would desirably possess is that it would allow for the slag tap diameter to be designed for slag flow considerations such as slag viscosity and velocity. A third such characteristic which such a new and improved slag tap system would desirably possess is that it take the form of sections, such that the sections can be unfastened one from another and be pushed along the floor of the gasifier vessel in order to thereby provide access to the interior of the gasifier vessel using the neck of the slag tap as a manway. A fourth such characteristic which such a new and improved slag tap system would desirably possess is that studded tubing would be employed over the slag touched surfaces so as to thereby allow the slag to solidify to form a heat transfer barrier as well as a corrosion and erosion resistant surface to protect the parent steel of the slag tap. A fifth such characteristic which such a new and improved slag tap system would desirably possess is that it would embody a stainless steel drip edge thereby allowing for good slag separation. A sixth such characteristic which such a new and improved slag tap system would desirably possess is that the water inlet and outlet piping thereof would be withdrawn from the tap hole, i.e., away from the drip edge area, thereby eliminating any excess tube surface near the drip edge on which slag could collect. A seventh such characteristic which such a new and improved slag tap system would desirably possess is that the slag tap sections be capable of being shop fabricated and of being later installed in the field with a minimum amount of field installation.
It is, therefore, an object of the present invention to provide a new and improved slag tap system for a gasifier.
It is another object of the present invention to provide such a new and improved slag tap system for a gasifier wherein the slag tap system is of self-supporting construction.
It is still another object of the present invention to provide such a new and improved slag tap system for a gasifier wherein the slag tap system permits the slag tap diameter to be designed for slag flow considerations such as slag viscosity and velocity.
Another object of the present invention is to provide such a new and improved slag tap system for a gasifier wherein the slag tap system is in the form of sections, such that the sections can be unfastened one from another and be pushed along the floor of the gasifier vessel in order to thereby provide access to the interior of the gasifier vessel using the neck of the slag tap as a manway.
A still another object of the present invention is to provide such a new and improved slag tap system for a gasifier wherein studded tubing is employed over the slag touched surfaces so as to thereby allow the slag to solidify to form a heat transfer barrier as well as a corrosion and erosion resistant surface to protect the parent steel of the slag tap.
A further object of the present invention is to provide such a new and improved slag tap system for a gasifier wherein the slag tap system embodies a stainless steel drip edge thereby allowing for good slag separation.
Yet an object of the present invention is to provide such a new and improved slag tap system for a gasifier wherein the water inlet and outlet piping thereof is withdrawn from the tap hole, i.e., away from the drip edge area, thereby eliminating any excess tube surface near the drip edge on which slag can collect.
Yet a further object of the present invention is to provide such a new and improved slag tap system for a gasifier wherein the slag tap sections of the slag tap system are capable of being shop fabricated and of being later installed in the field with a minimum amount of field installation.
Yet another object of the present invention is to provide such a new and improved slag tap system for a gasifier which is advantageously characterized in that it is suitable for employment in newly constructed gasifiers as well as being equally suitable for utilization in gasifier retrofit applications.
Yet still another object of the present invention is to provide such a new and improved slag tap system for a gasifier which is advantageously characterized in that it is relatively inexpensive to provide, yet despite being relatively simple in construction is reliable in operation.